[Regression] Core Location underground positioning inaccurate on iOS 26.1 beta (23B5044i)

Question

Created 6h

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Summary

While parallel testing Core Location on the new iOS 26.1 beta (23B5044i), I observed what I believe to be a regression of the issue described here: https://developer.apple.com/forums/thread/779192

Specifically, user positioning underground subway stations is noticeably inaccurate on the beta, whereas the same scenarios remain accurate on the unupgraded device below.

I work with the MTA (New York City) and work with the OP of that thread. Happy to provide additional testing or details if helpful. Please let me know what else you need.

Test Info

Riding NYCT from Wall St to 34th St Penn Station on the 2 train carrying two iphones
Recording: https://limewire.com/d/dpTWi#pDC3GRYIdE
Expected: Consistent underground positioning comparable to prior releases.
Actual: Degraded/inaccurate underground positioning on iOS 26.1 beta.

Test Devices

Left Screen: iPhone 15 Pro Max - iOS 26.1 beta (23B5044i)
Right Screen: iPhone 11 - iOS 18.6.2 (22G100)

Blue dots show location set by CoreLocation. Red dot on iphone 11 shows the actual location of both devices as I was able to manually place while travelling through a station. Placement through tunnels is not easy to verify and not usually indicated.

Timestamps

Comparison of when train was actually observed in a station vs when 26.1 and 18.6.2 CoreLocation updated to the station

Fulton St

1:48 iOS 26.1 correctly updates (correctly)
2:16 iOS 18.6.2 updates (28sec late)

Park Place

4:12 train arrives
4:15 iOS 18.6.2 updates to ~near Park Place
5:04 iOS 18.6.2 updates to Park Place (correctly)
6:07 iOS 26.1 update to ~near Park Place (over 2 mins late)

Chambers St

6:02 train arrives / iOS 18.6.2 updates (correctly)
6:14 iOS 26.1 updates to ~near Chambers
6:18 iOS 26.1 update to Chambers (correctly)

Franklin St

6:52 train arrives
6:55 iOS 18.6.2 updates (correctly)
x:xx iOS 26.1 does not update

Canal St:

7:16 train arrives
7:18 iOS 18.6.2 updates (correctly)
x:xx iOS 26.1 does not update

Houston St

7:54 train arrives
8:00 iOS 18.6.2 updates (correctly)
x:xx iOS 26.1 does not update

Christopher St

8:37 iOS 26.1 presumably between Houston St and Christopher St
8:40 train arrives / iOS 18.6.2 updates (correctly)
x:xx iOS 26.1 does not update

14 St

9:22 train arrives
9:28 iOS 18.6.2 updates (correctly)
11:01 as train departs station iOS 26.1 updates (1.5 mins late)

Boost

Answer 1

Engineer OP

Apple

1h

Please file a bug report about this issue with 26.1 right away.

We would also need diagnostic logs, It would be very helpful if you could please go to https://developer.apple.com/bug-reporting/profiles-and-logs/ and follow the instructions for Location Services for iOS to install a logging profile on your devices. Then reproduce the issue, and follow the instructions at the above link to create a sysdiagnose for both devices. And attach that to the Feedback report as well.

To file a Feedback Report, you can use the Feedback Assistant.

Once you have done so, please share the Feedback ID on this thread so I can make sure the team is aware of it.

Once the Location team is on the case, they may contact you for further information or specific test cases.

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